1. Field of the Invention
The invention relates to a system for inspecting matt, flat and/or slightly curved surfaces in order to identify defects which are associated with a modification of the course of the surface, especially for checking flat/unlacquered shell bodywork.
2. Discussion of Related Art
Systems for inspecting matt, flat and/or curved surfaces in order to identify surfaces are known, which have surfaces radiating strip-shaped light, by means of which surfaces unlacquered metal and plastics material surfaces are sampled for topographical defects in a direction parallel with the surface. This requires incident light radiation at very flat radiation angles of approximately 5 to 10xc2x0, because at these angles the surfaces reflect in a very directed manner, whilst at steeper angles they predominantly reflect in a diffuse manner. Such illumination strips are suitable only for small surface portions, or means have to be provided with which the strips can be moved relative to the surface (WO 98/15815). If, however, a plurality of strips are lined up beside one another to illuminate larger surfaces, surface portions of the surface to be sampled which are located closer to the lights are illuminated at undesirably steep angles. This leads to masking of defects and identifiable colours and colour effects which would normally be recognisable when associated with the luminous strip. Furthermore the problem of direct dazzling occurs since the lights do not only radiate in the desired directions onto the surface as a result of their light distribution which is too wide.
The object underlying the invention, therefore, is to create a system for inspecting matt, flat and/or slightly curved surfaces which always creates the same illumination conditions over a surface of any size to be sampled, the light distribution of the illumination being intended to be suitable for making topographical defects of an otherwise continuous surface clearly recognisable, the light radiation being intended to be limited to the solid angle required to illuminate the surface, in order to avoid dazzling when the light-radiating surface is viewed directly. At the same time, the luminous device used should be simple in its structure.
This object is accomplished according to the invention by the features of the main claim.
According to the invention the illumination device comprises a plurality of elongated luminous surfaces, disposed substantially parallel to one another and possessing substantially all the same light distribution, which is tightly concentrated in planes which lie transversely with respect to the longitudinal direction of the surfaces, with an aperture angle which is smaller than 15xc2x0, preferably 5xc2x0, and by even greater preference smaller than 2xc2x0, such that a substantially sheet-form light distribution is produced which covers the surface element to be inspected on the surface. In planes in the longitudinal direction of the surfaces, the illumination device has in each case a widely radiating light distribution. The illumination device is so disposed that the angle between the normal line of the inspected surface element and the connecting line between the inspected surface element and any point on the elongated luminous surface is greater than roughly 60xc2x0, preferably however 75xc2x0, and the observer, i.e. a person or a camera or the like, is located within or at least in the proximity of the at least one elongated strip illuminating by reflection of the sheet-type light distribution on the surface portion to be inspected. By means of this system, illumination is achieved which does not cause any inadvertent dazzling when the light-radiating surfaces are viewed directly, and which permits good detectability of defects which are connected with a modification of the course of the surface.
With the same illumination principle, the position of the observer can be altered by optical means such as mirrors, retro-reflective foils or prisms.
Through the measures quoted in the subordinate claims, advantageous developments and improvements are possible.
Preferably the illumination device used has a light-radiating original surface, in front of which a bundle with parallel lamellae surfaces is disposed, the main radiation direction of the original surface radiating light through the lamellae arrangement. The original surface has uniform luminance distribution and radiates light in a larger solid angle area than the totality of the elongated light-radiating surfaces at the end of the lamellae bundle. The lamellae arrangement is preferably so dimensioned in respect of its geometry, i.e. spacing and depth, that diagonally to the lamellae surface the aperture angle is smaller than 15xc2x0, preferably smaller than 5xc2x0, and by particular preference smaller than 2xc2x0.
The original surface is preferably formed by elongated light sources lying beside one another or also by at least one elongated light source which is surrounded by a trough-like reflector. The elongated lamps can be for example fluorescent lamps or linear incandescent lamps, the light distribution of which is automatically widespread in planes parallel to the lamp axis. It is propitious to use only a few lamps and so to align their light distribution by reflectors or lenses that the solid angle region over which the radiation falls is greater than the solid angle required by the entire illumination device.
According to the invention, the surfaces of the lamellae scatter light in a diffuse manner with a is smaller proportion of directed reflection, or they reflect in a directed manner with a small reflection factor. A gap-free illumination in a narrow angle range is produced according to the invention with surfaces which at very flat incident light angles have a high degree of directed reflection and at steeper incident light angles pass into diffuse reflection. These are, for example, lacquered surfaces or metallic surfaces. The surfaces of the lamellae are preferably black or grey.
In a preferred embodiment, the lamellae can also be disposed either on one or on both sides of light guide plates so as to be optically dense, filling the gaps between the plates and having polished light entrance and exit surfaces.